The objective of homogenizing a glass melt is to reduce spatial and temporal variations in the chemical composition of the glass melt, in accordance with the product requirements. Chemical inhomogeneities can result in inhomogeneities in the refractive index, which may impair optical depiction, for example, and in inhomogeneities in the viscosity, which may result, for example, in uncontrolled geometrical variations during hot finishing processes or hot processing. To this end, a differentiation is made between macro-inhomogeneities, which is to say a variation in chemical composition on comparatively large spatial scales of for example, a few centimeters having small spatial gradients, and micro-inhomogeneities (also referred to as striations), which is to say a variation of the chemical composition on small spatial scales of, for example, 0.1 to 2 nm having in part large spatial gradients. The goal of the homogenizing process is to eliminate macro-inhomogeneities and micro-inhomogeneities to as great an extent as is possible so that, for example, a smooth progression of the refractive index can be obtained.
Glass melts are characterized in that, in typically used stirring systems, they have viscosities ranging between 1 and 200 Pa·s, which results in laminar flow of the glass melt (Reynolds number<1), and in that the chemical coefficient of diffusion is generally less than 10−12 m2/s, so that homogenization that can be achieved by way of diffusion is negligibly small. Rather, homogenization of glass melts can generally only be achieved by considerably expanding, redistributing and chopping local inhomogeneities and/or striations. For this purpose stirring systems are used, which comprise a melt receptacle for temporarily receiving the glass melt and at least one stirring device for stirring the glass melt in the melt receptacle.
In order to achieve any suitable homogenization under the above conditions, particularly with high viscosities and small chemical coefficients of diffusion, the gap between the stirrer blades of the stirring device and the wall of the melt receptacle is conventionally kept to a minimum. An excessively narrow gap between the stirrer blades and the melt receptacle wall, however, poses the risk of the stirrer coming into contact with the wall of the melt receptacle, with consequent damage to the stirrer and/or the stirrer vessel. Here, it must be remembered that the stirrer can only ever be adjusted when the melt receptacle is in a cooled state. Because thermally induced deformation of the stirrer or of the stirring system are unavoidable when heating to operating temperatures, the adjustment of the components is often no longer correct at the operating temperatures. This can result in an excessively narrow distance between the stirrer blades and the melt receptacle wall, and thus in direct contact with the material, which ultimately results in destruction of the stirring system.
The relative marginal gap width, i.e. the ratio 0.5*(diameter of the stirring device or diameter of the melt receptacle minus diameter of the stirrer)/(diameter of the stirring device or of the melt receptacle), is typically less than approximately 5%, or even less than approximately 1%, of the melt receptacle diameter or of the diameter of the stirring device. Due to the aforementioned thermal deformation of the components when heating the device to the operating temperature, the width of the gap cannot be consistently maintained, so that large marginal gaps must typically be specified. For this reason, only unsatisfactory homogenization results are achieved with the state of the art, particularly for high-viscosity glass melts.
High shear stress between the stirrer blades and melt receptacle wall due to a narrow marginal gap can considerably impair the service life of the stirring system. In addition, there is the risk that, if the stirring gap is excessively narrow, bubbles adhering to the melt receptacle wall may be sheared off and transferred into the product. High shear stresses can also bring about abrasion of the wall material of the melt receptacle or stirrer vessel, resulting in micro-inclusions in the glass or the glass ceramic, which are not desirable, particularly in display glass products.
US 2003/0101750 A1 discloses a method and a device for homogenizing a glass melt for the production of display glass. At a predefined stirring efficiency, which is determined by the stirrer diameter, stirrer speed and marginal gap, a predefined shear rate is selected. The marginal gap is comparatively narrow and corresponds approximately to 6% to 9% of the free diameter of the stirrer vessel.
Furthermore homogenization can also be achieved by the geometry of the actual stirrer blades. The inclination of the stirrer blades and hence the feed action of the stirrer are preferably set such that the blades operate counter to the glass flow in the glass melt receptacle. To this end, an axial feed action can be achieved by the angle of the stirrer blades, by the geometric shape of the stirrer blades and/or by a helical arrangement of the stirrer blades on the stirrer shaft. For example, JP 10265226 A discloses a configuration, wherein, the inner stirrer blades feed downward, while the outer stirrer blades feed upward so as to achieve improved homogenization. JP 63008226 A discloses that the inclination of the stirrer blades, and hence the feed action of the stirrer, can be adjusted so that the blades operate counter to the glass flow. In this way, dead space in the glass melt receptacle should be avoided.
For the reasons given above, according to the state of the art, the smallest possible marginal gap is always desirable with a view to achieving the highest possible homogeneity.
U.S. Pat. No. 2,831,664 discloses a method and a device for homogenizing a glass melt, comprising a stirring device having a plurality of stirrer blades axially offset in relation to one another. The stirring device is disposed in a cylindrical stirrer pot, which is provided with an inlet for the glass melt at an upper edge and an outlet for the glass melt at the lower end. In a marginal gap between the inside wall of the stirrer pot and the stirrer blades the stirrer blades form a plurality of regions having radial and at the same time vertical glass flow. The dimensions of the stirring device produce a very narrow marginal gap, resulting in very high material stresses caused by the very high shear rates that are applied.
JP 2001-72426 A and the English abstract thereof disclose a device for homogenizing a glass melt. The stirring device is disposed in a cylindrical stirrer pot, which is provided with an inlet for the glass melt at an upper end and an outlet for the glass melt at the lower end. The glass flows in the marginal gap between the inside wall of the stirrer pot and the stirrer blades and also in the stirrer circuit are flows flowing in the same direction in relation to the superimposed throughput flow. This results in a comparatively poor homogenization result.
US 2002/0023464 A1 discloses a device for homogenizing a glass melt, comprising a centerline recirculation channel, specifically on the inside of the mixing shaft, or a separate external recirculation channel. The glass melt consequently does not flow back in a marginal gap as defined by the present invention. A very narrow gap between the inside wall of the stirrer pot and the mixing blades is disclosed, which produces a very high mechanical load on the stirrer and the stirrer vessel.
US 2003/0101750 A1 discloses a method, which is modified compared to the aforementioned U.S. Pat. No. 2,831,664, wherein the disadvantage of a very narrow marginal gap is mitigated in that the stirrer system is enlarged almost to scale in order to guarantee homogeneity with increased mass throughput. This is achieved either by increasing the rotational speed or by enlarging the stirrer volume. A rotational speed increase, however, brings about an increased shear rate and thus a higher precious metal exposure level, including the undesirable generation of precious metal particles in the stirrer vessel. An enlarged stirrer volume is associated with higher material use and costs.
Both solutions are mathematically defined with the help of a non-dimensional homogeneity number H, which defines the homogenization potential of the stirring device. It is apparent that, at a fixed homogeneity number H and predefined throughput, the rotational speed of the stirring device is considered in a linear fashion and the size of the stirrer system is considered, in the case of geometric similarity, only with the reciprocal third root (cubic root). A desired homogenization level can thus be implemented much more easily with the help of a to-scale enlargement of the stirrer system than with a rotational speed increase, particularly since a rising rotational speed increases the shear forces and material stress or particle abrasion in the marginal gap.